The objective of this study is to study the effects of low levels of SO2 and respirable particulates on the health of human beings and to identify whether there is a degree of interaction between these pollutants at these levels. Communities will be selected that have different levels of SO2 and respirable particulates. Random samples will be selected from the adult population (25-74) and studied by means of a standard respiratory questionnaire and simple tests of pulmonary function. These populations will be re-examined at 3-4 year intervals using the same methods. Occupational history and tobacco smoking habits will be obtained and used in the analyses. Levels of air pollution will be obtained from the air monitoring network and will be augmented by our own aerometric studies in these communities. Persons who die will be recorded and deaths will be analyzed by over-all mortality and cause specific mortalities. Children in the first and second grades will also be studied by means of simple tests of pulmonary function. These will be followed annually and a new cohort of the new first grade will be added to the sample each year. Limited information on respiratory symptoms will be obtained and in those over the fourth grade cigarette smoking habits will be asked about and recorded.